Do these two questions answer each other? The Trace may not be on wandless magic; the trace is on the magical signature from when a spell is produced from a wand. (And Dobby deliberately ‘forges’ a magical signature onto his spell to incriminate Harry; Dobby’s other uses of magic therefore go unnoticed) The Lumos was Harry’s desperation forcing a use of wandless magic; it wasn’t a regular spell.
The incident with Aunt Marge would seem to refute this theory.